Atomic and magnetic structures, disorder effects, and unconventional superexchange interactions in A2MnGaO5+δ (A=Sr, Ca) oxides of layered brownmillerite-type structure

Crystal and magnetic structures of complex manganese oxides Sr2GaMnO5+δ (δ≅0.01,0.52) and Ca2GaMnO5+δ (δ≅0.05) were studied by neutron powder diffraction (ND) and μSR technique in the temperature range 2-300 K. The crystal structures contain single MnO2 layers separated by three nonmagnetic cation-oxygen layers. The principal difference between the δ≅0 and δ≅0.5 compounds is the Mn valence: Mn3+ or Mn4+, and the structure of the (GaO1+δ) buffer layer, which is formed by tetrahedra or partially filled octahedra, respectively. The magnetic moments of the manganese ions are coupled antiferromagnetically in the MnO2 plane, but antiferromagnetically (G type) or ferromagnetically (C type) between the planes for the reduced and oxidized compositions, respectively. The transition from the G- to C-type magnetic structure by oxygen doping is explained by strong diagonal 180 deg. superexchange antiferromagnetic interaction between Mn4+-ions in the adjacent layers through additional oxygen atoms in the GaO buffer layer. The magnetic moments in Sr-based samples are appreciably reduced in comparison with the spin-only values of the corresponding Mn ion. By using complementary information on local magnetic field distribution from μSR we show that the reduced magnetic moments seen by ND are caused by the presence of locally flipped Mn spins and a short-ranged (40 A) antiferromagnetic phase. The magnetic disorder can be caused by the disorder observed in the oxygen positions of the GaO1+δ layer, because the coupling between the MnO2 layers is mediated by the geometry of the superexchange path through these oxygen atoms